Aromatic hydrocarbons such as benzene, toluene and xylene (collectively referred to in the industry as “BTX” or “light aromatics”) play an important role in the chemical industry. For example, benzene is used to manufacture plastics, lubricants, rubbers, synthetic fibers and dyes. Toluene is used as a solvent for paint thinners, nail polish remover, glues, adhesives, rubber and disinfectants. Xylene is also used to produce plastics and synthetic fibers.
Aromatic hydrocarbons are derived from petroleum via various processes, such as catalytic reforming of naphtha or fluid catalytic cracking of heavy gas oil. Light alkanes can undergo a series of reactions when catalyzed by crystalline aluminosilicate and metals such as Pt, Ga, and/or Zn and produce industrially valuable aromatic hydrocarbons. Measurable quantities of methane, hydrogen, light alkanes, and light alkenes are also produced as by-products. Due to its plentiful presence and low cost driven by shale gas production, light alkanes provide opportunity for aromatic hydrocarbons production. However, there remain a number of technical obstacles for industrial scale production of aromatic hydrocarbons from light alkanes.
Light alkane aromatization is a strongly endothermic reaction, and therefore requires a large quantity of reaction heat supply at relatively high temperature. However, preheating the light alkane feedstock to provide sufficient sensible heat for the endothermic reaction is not feasible because the reaction heat required for industrially attractive rates is substantially larger than the quantity of sensible heat achievable through feedstock preheating. Excessive preheating of the feedstock in an attempt to increase its sensible heat and provide reaction heat may lead to a number of technical issues, including thermal breakdown of feedstock and shortened lifetime of preheating tubes. In addition, intensive heating-up of reactor tubes with small diameter in a furnace, which is conventionally used in steam methane reforming plants, would not be suitable for the light alkane conversion to aromatic hydrocarbons because its conversion rate is relatively low. Moreover, heating of reactor tubes packed with catalyst often creates undesirable temperature distribution in the catalyst bed, which leads to accelerated catalyst coking and sintering problems causing reversible or irreversible loss of catalyst activity. This imposes a burden on production cycle, equipment operation, system control, and overall process efficiency, and lowers aromatic hydrocarbons yield. A conventional way to regenerate deactivated catalyst is to burn off the coke with air or diluted air while aromatic hydrocarbons production process is stalled; hence, making it impossible to produce aromatic hydrocarbons in a continuous manner.
Taken together, there is a need for a new and improved process for producing aromatic hydrocarbons from light alkane feedstock by developing a uniform and reliable reaction heat supply to the catalyst bed and by making the entire process continuous.